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Electroencephalogram (EEG)

Test Overview

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors (electrodes) are attached to your head and hooked by wires to a computer. The computer records your brain's electrical activity on the screen or on paper as wavy lines. Certain conditions, such as seizures, can be seen by the changes in the normal pattern of the brain's electrical activity.

Why It Is Done

An electroencephalogram (EEG) may be done to:

  • Diagnose epilepsy and see what type of seizures are occurring. EEG is the most useful and important test in confirming a diagnosis of epilepsy.
  • Check for problems with loss of consciousness or dementia.
  • Help find out a person's chance of recovery after a change in consciousness.
  • Find out if a person who is in a coma is brain-dead.
  • Study sleep disorders, such as narcolepsy.
  • Watch brain activity while a person is receiving general anesthesia during brain surgery.
  • Help find out if a person has a physical problem (problems in the brain, spinal cord, or nervous system) or a mental health problem.

How To Prepare

Before the day of the electroencephalogram (EEG) test, tell your doctor if you are taking any medicines. Your doctor may ask you to stop taking certain medicines (such as sedatives and tranquilizers, muscle relaxants, sleeping aids, or medicines used to treat seizures) before the test. These medicines can affect your brain's usual electrical activity and cause abnormal test results.

Do not eat or drink foods that have caffeine (such as coffee, tea, cola, and chocolate) for 12 hours before the test.

Since the electrodes are attached to your scalp, make sure your hair is clean and free of sprays, oils, creams, and lotions. Shampoo your hair and rinse with clear water the evening before or the morning of the test. Do not put any hair conditioner or oil on after shampooing.

To find certain types of abnormal electrical activity in the brain, you may have to be asleep during the recording. You may be asked not to sleep at all the night before the test or to sleep less (about 4 or 5 hours) by going to bed later and getting up earlier than usual. If your child is going to be tested, try to keep him or her from taking naps just before the test. If you know that you are going to have a sleep-deprived EEG, plan to have someone drive you to and from the test.

How It Is Done

An electroencephalogram (EEG) may be done in a hospital or in a doctor's office by an EEG technologist. The EEG record is read by a doctor who is specially trained to diagnose and treat disorders affecting the nervous system (neurologist).

You will be asked to lie on your back on a bed or table or relax in a chair with your eyes closed. The EEG technologist will attach several flat metal discs (electrodes) to different places on your head, using a sticky paste to hold the electrodes in place. A cap with fixed electrodes may be placed on your head instead of individual electrodes. In rare cases, these electrodes may be attached to the scalp with tiny needles.

The electrodes are hooked by wires to a computer that records the electrical activity inside the brain. A machine can show the activity as a series of wavy lines drawn by a row of pens on a moving piece of paper or as an image on the computer screen.

Lie still with your eyes closed during the recording, and do not talk to the technologist unless you need to. The technologist will watch you directly or through a window during the test. The recording may be stopped from time to time to allow you to stretch and reposition yourself.

The technologist may ask you to do different things during the test to record what activity your brain does at that time.

  • You may be asked to breathe deeply and rapidly (hyperventilate). Usually you will take 20 breaths a minute for 3 minutes.
  • You may be asked to look at a bright, flashing light called a strobe. This is called photic or stroboscopic stimulation.
  • You may be asked to go to sleep. If you can't fall asleep, you may be given a sedative to help you fall asleep. If an EEG is being done to check a sleep problem, an all-night recording of your brain's electrical activity may be done.

An EEG takes 1 to 2 hours. After the test, you may do your normal activities. But if you were sleep-deprived or given a sleep medicine, have someone drive you home after the test.

How It Feels

There is no pain with an electroencephalogram (EEG).

If paste is used to hold the electrodes, some paste may stay in your hair after the test, so you will have to wash your hair to remove it. If needle electrodes are used (which is rare), you will feel a brief, sharp prick (about like having a hair pulled out) when each electrode is inserted. If electrodes are placed in your nose, they may cause a tickling feeling and, rarely, some soreness or a small amount of bleeding for 1 to 2 days after the test.

If you are asked to breathe rapidly, you may feel lightheaded or have some numbness in your fingers. This reaction is normal. It will go away a few minutes after you start breathing normally again.

Risks

An electroencephalogram (EEG) is a very safe test. The electrical activity of your brain is recorded, but at no time is any electrical current put into your body. An EEG should not be confused with electroshock (electroconvulsive) therapy.

If you have a seizure disorder such as epilepsy, a seizure may be triggered by the flashing lights or by hyperventilation. If this occurs, the technologist is trained to take care of you during the seizure.

Results

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors (electrodes) are attached to your head and hooked by wires to a computer. EEG test results are ready on the same or the next day.

There are several types of brain waves:

  • Alpha waves have a frequency of 8 to 12 cycles per second. Alpha waves are present only in the waking state when your eyes are closed but you are mentally alert. Alpha waves go away when your eyes are open or you are concentrating.
  • Beta waves have a frequency of 13 to 30 cycles per second. These waves are normally found when you are alert or have taken high doses of certain medicines, such as benzodiazepines.
  • Delta waves have a frequency of less than 3 cycles per second. These waves are normally found only when you are asleep or in young children.
  • Theta waves have a frequency of 4 to 7 cycles per second. These waves are normally found only when you are asleep or in young children.
Electroencephalogram (EEG)
Normal:

In adults who are awake, the EEG shows mostly alpha waves and beta waves.

The two sides of the brain show similar patterns of electrical activity.

There are no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.

If flashing lights (photic stimulation) are used during the test, one area of the brain (the occipital region) may have a brief response after each flash of light, but the brain waves are normal.

Abnormal:

The two sides of the brain show different patterns of electrical activity. This may mean a problem in one area or side of the brain is present.

The EEG shows sudden bursts of electrical activity (spikes) or sudden slowing of brain waves in the brain. These changes may be caused by a brain tumor, infection, injury, stroke, or epilepsy. When a person has epilepsy, the location and exact pattern of the abnormal brain waves may help show what type of epilepsy or seizures the person has. Keep in mind that in many people with epilepsy, the EEG may appear completely normal between seizures. An EEG by itself does not diagnose or rule out epilepsy or a seizure problem.

The EEG records changes in the brain waves that may not be in just one area of the brain. A problem affecting the entire brain—such as drug intoxication, infections (encephalitis), or metabolic disorders (such as diabetic ketoacidosis) that change the chemical balance in the body, including the brain—may cause these kinds of changes.

The EEG shows delta waves or too many theta waves in adults who are awake. These results may mean brain injury or a brain illness is present. Some medicines can also cause this.

The EEG shows no electrical activity in the brain (a "flat" or "straight-line" EEG). This means that brain function has stopped, which is usually caused by lack of oxygen or blood flow inside the brain. This may happen when a person has been in a coma. In some cases, severe drug-induced sedation can cause a flat EEG.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Moving too much.
  • Taking some medicines, such as those used to treat seizures (antiepileptic medicines) or sedatives, tranquilizers, and barbiturates.
  • Not eating before the test.
  • Drinking coffee, soda, tea, or other foods that contain caffeine before the test.
  • Being unconscious from severe drug poisoning or a very low body temperature (hypothermia).
  • Having hair that is dirty, oily, or covered with hair spray or other hair preparations. This can cause a problem with the placement of the electrodes.

What To Think About

  • If the doctor thinks that a person has epilepsy but the EEG is normal, the technologist running the EEG test may have the person look at a flashing light (photic stimulation), breathe fast and deeply (hyperventilation), or sleep during the test. These techniques sometimes show epileptic EEG patterns that did not show up at first. If epilepsy is suspected after an initial EEG, the doctor may repeat the EEG more than once.
  • An EEG done during a seizure will almost always show abnormal electrical patterns. This makes an EEG useful when a doctor thinks that a person is having psychogenic nonepileptic seizures (also called pseudoseizures), which have no physical cause but can be caused by stress, emotional trauma, or mental illness. Psychogenic seizures do not cause abnormal electrical activity in the brain and will not show abnormal EEG results.
  • Other tests that may also be done include:
    • Video EEG. Video EEG records seizures on videotape and on computer so that the doctor can see what happens just before, during, and right after a seizure. This test can be very helpful in finding the specific area of the brain that the seizures may be coming from. It is also helpful in diagnosing psychogenic seizures, which may look like real seizures but do not affect the electrical activity in the brain. Video EEG may be used short-term or long-term:
      • Short-term monitoring is done on an outpatient basis and may last up to 6 hours.
      • Long-term monitoring is done in the hospital and may last 3 to 7 days.
    • Brain mapping. Brain mapping is a fairly new method that is very similar to EEG. With electrodes placed on the person's scalp to transmit the brain's electrical activity, a computer makes a color-coded map of signals from the brain. It is sometimes done to find a specific problem area in the brain that has already shown up on a regular EEG. Doctors are still not certain how brain mapping could be best used.
    • Ambulatory EEG monitoring. In ambulatory EEG monitoring, the person is able to move around, and the test allows for long periods of time in recording of electrical activity in the brain. Fewer electrodes are attached to the person, and the person carries a small, portable recording unit. The recording may last for a full day or more, and the person is allowed to leave the hospital. Ambulatory EEG monitoring is not as accurate as a regular EEG.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Credits

By Healthwise Staff
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer Colin Chalk, MD, CM, FRCPC - Neurology
Last Revised May 8, 2012

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